1. Field of the Invention
The present invention, disclosed hereinafter, concerns generally an apparatus and method for reducing signal errors attendant to pulse or digital communications transmitted along lossy, band-limited, time-invariant, fixed length, single channel transmission path. In particular, the present invention provides apparatus and method for reducing the error-rate of digital PCM (pulse code modulation) data by means of preemphasis precoding or encoding of the binary PCM data signal prior to transmission on a single channel of a cable system.
2. Description of the Related Art
The electronic transmission of data and information by means of pulse and digital systems typically involves either a variable path, such as a radio link between a cellular or cordless is telephone and a repeater node, or alternatively, a fixed, time variant path, such as a copper fiber optic cable connecting a cable television subscriber's receiver to a provider's distribution transmitter.
The present invention concerns the latter fixed path. For fixed path cable transmissions, the fidelity, i.e. veracity, of the transmitted data or information is limited by the bandwidth characteristics (also called transfer characteristics, impulse response) of the equivalent, passive network representing each channel of the transmission line. In many cases, a transmission line for pulse and digital communications can feature multiple channels, permitting separate clocking and error correction channels for each data channel; however, in some cases, exigent economic and physical demands prescribe a minimal size, single channel cable or path for each data or information stream. These constraints foreclose the option of a separate clocking or timing channel. Also, there may be constraints to the low frequency response of the designed transmission system. To remedy or mitigate these channel availability and frequency response constraints bi-phase level, self clocking PCM techniques are commonly employed. Unfortunately, such codes effectively reduce the available data and information bandwidth by half. These issues and associated remedies are discussed in “Digital Recording Methods”, Magnetic Tape Recording Technical Fundamentals, Datatape Incorporated, 1976, pages 9599, incorporated by reference herein for purposes of indicating the background of the invention and illustrating the mature state of the art. The rationale and utilization of bi-phase PCM technology is well known to those practicing the art.
FIG. 1 illustrates a typical prior art, single channel, digital PCM data transmission system.
Biphase PCM Waveforn-21 is the digital PCM data input to Buffer AmplifieI22, representing a simple transmitter, comprising either digital or analog circuitry. A Fixed Transmission Line 23, featuring a single conductor per channel, each channel exhibiting both resistive and frequency dependent attenuation, conducts the output signal from Buffer Amplifier 22 to the input of simplified, typical Receiver 24, comprising Wide-band Amplifier25 and Multipole Equalization Network 26. Wide-band Amplifier25, employing either analog or digital circuitry, is used to compensate for the attenuation of Waveforri21 upon transmission through the lossy Fixed Transmission Line 23. Output from Wide-Band Amplifier 25 thereupon goes to the input of Multipole Equalization Network 26. Although typically an active network Multipole Equalization Network 26 can also be a passive network. Intended to reconstruct the degraded fidelity of the signal present at the input to Receiver 24 to a replication of Waveform 21 by compensating for the frequency dependent losses attendant to the Fixed Transmission Line 23, Network 26 tends to be complex and difficult to implement. Although Network 26 may also include some techniques to correct the slope or rise time of the attenuated received signal, such techniques are often primitive and inadequate for reliable, high-quality digital PCM data transmission. This inadequacy typically is remedied by Bit Synchronizer 27, which processes the output from Multipole Equalization Network 26, correcting for slope and rise time degradation, and producing as an output, Reconstructed PCM Waveform28, which is desired to be a replication of Bi-phase PCIVI Waveform 21 of sufficient veracity and reliability to fulfill the intended purpose of the data transmission system. Unfortunately, as illustrated in FIG. 1, Bit Synchronizer 27 is normally a separate, often necessary, add-on component to the typical receivef24. Containing expensive, complex circuitry, this synchronizer component can cost thousands to tens of thousands of dollars; proper utilization often entails lengthy, difficult adjustments,